The problem of icing on aerofoils is well known in the aeronautics industry. The shape of the aerofoils may be altered on account of the formation of ice that occurs because during flight, the aerofoil encounters droplets of supercooled water contained in the atmosphere.
This problem is often dealt with by equipping the aerofoil with a Joule-effect heating structure. A distinction is made between "deicers", in which the resistive elements that dissipate heat are powered intermittently to remove the ice that regularly forms, and "anti-icing devices", in which the resistive elements are powered continuously to prevent ice from forming. Although the present invention is explained hereafter in its application to anti-icing devices, by way of an illustration, it will be understood that it is also applicable to deicers.
Usually, the heating structure consists of metal resistors. These metal resistors pose problems of mechanical integrity, particularly for aerofoils that are made of composite material, of tolerance to damage (multiple redundancy is needed to ensure that the breakage of one metal resistor does not prevent the entire device from functioning), of uniform heating per unit area, and of corrosion.
In order to limit the occurrence of these problems, it has been proposed that use be made of a composite deicer in which the resistive elements are composed of carbon fibers (see French Patent 2 578 377). These resistive elements form strips of carbon fibers which preferably run along the leading edge of the aerofoil.
The invention finds a particular application in the field of helicopter blades.
A number of complex physical phenomena are involved, often in conflicting ways, in the thermal behavior, the accumulation and the deicing of a blade. To make the explanations clear, these will be limited here to describing the simplest phenomena, and they will be considered independent of one another:
a) there is more kinetic heating caused by the rotation of a blade in the air at its tip than there is at the root of the blade. This means that the power to be provided per unit area to give thermal protection tends to decrease toward the blade tip; PA1 b) the convective heat exchange coefficients are higher at the blade tip because of the higher rotational speed. With all else being equal, furthermore, this phenomenon means that the power to be provided per unit area for thermal protection is higher at the tip; PA1 c) the accumulation volume is greater at the blade tip because the particles of supercooled water have less tendency to be deflected by the aerodynamic flow around the aerofoil when the relative speeds increase. Taken in isolation, this phenomenon requires a higher thermal power per unit area at the blade tip; PA1 d) simulations (codes and testing) of the thermal behavior of an anti-iced blade section show that under certain flight conditions (flying in dry air with the anti-icer running), the temperatures reached within the blade exceed the permissible limits for the resins of which this blade is made. These codes are then used to solve the opposite problem and calculate the thermal power per unit area that is permissible in order not to exceed these "critical" temperatures for the resins. The change obtained in the thermal power per unit area is the opposite to case "a" above because the phenomenon in dry air is governed by convective exchanges, which leads to a reduction in the heating power per unit area which is greater at the blade root than at the tip.
It should be noted that in general, the pilot does not have a sufficiently reliable detector of ice formation available to him. Thus, when he finds himself in conditions that may cause icing (finds himself approaching icing clouds, etc.), he switches on the anti-icing device in anticipation: the air may still be dry though. Under these conditions, there is the risk that the anti-icing device will be too effective and will give rise to excessive blade temperatures. These conditions therefore need to be taken into consideration when designing the anti-icing device.
Taking the various relevant phenomena into account, the aerodynamicists and the thermodynamicists are laying down specifications regarding the change in thermal power per unit area to be provided over the surface of the aerofoil. In order to obtain a variation in thermal power per unit area perpendicular to the direction of the strips, it is possible to envisage thicknesses of carbon that differ from one strip to another. To obtain a variation parallel to a strip, common practice is to interrupt some of the layers of fibers of which the strip is formed at certain positions along the strip, which leads to thicknesses and electrical resistances that differ in successive portions. This approach causes sharp variations in power per unit area and does not readily allow the thermal specifications to be met.